1. Field of the Invention
The invention relates in general to a signal processing device and associated method, and more particularly to a signal processing device and associated method capable of determining whether a spectrum of a multicarrier signal is reversed.
2. Description of the Related Art
FIG. 1 shows a block diagram of a conventional orthogonal frequency division multiplexing (OFDM) baseband receiver for a digital television. The OFDM baseband receiver 10, suitable for Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) and Digital Video Broadcasting-Terrestrial (DVB-T) digital televisions, includes an antenna 105, a tuner 110, an analog-to-digital converter (ADC) 120, an automatic gain control (AGC) 130, a frequency offset compensation and spectrum reversion circuit 140, a time-domain processing circuit 150, a fast Fourier transform (FFT) circuit 160, and an integer carrier frequency offset (ICFO) estimation circuit 170. After receiving an RF signal from the antenna 105, the tuner 110 selects a target channel from multiple channels, and down-converts the RF signal of the target channel to an intermediate low frequency signal. The intermediate low frequency analog signal is converted by the ADC 120 to a digital signal that is then transmitted to the AGC 130 to adjust a gain of the signal. The frequency offset compensation and spectrum reversion circuit 140 performs frequency offset compensation on the digital signal. The time-domain processing circuit 150 at the next stage processes the digital signal by such as a synchronization process, such that a sampling period of symbols at the signal receiver end may be consistent with that at the signal transmitter end. The FFT circuit 160 converts the digital signal from time domain to frequency domain. The ICFO estimation circuit 170 estimates a carrier frequency offset of the digital signal. The frequency offset compensation and spectrum reversion circuit 140 performs offset frequency compensation on the digital signal according to an estimation result of the ICFO estimation circuit 170.
The ICFO estimation circuit 170 performs a correlation operation on the digital signal to obtain a plurality of correlation values, according to which the carrier frequency offset of the digital signal can be obtained. More specifically, the carrier frequency offset includes an integer carrier frequency offset (ICFO) and a fractional carrier frequency offset (FCFO), and the correlation values may be further categorized into ICFO correlation values and FCFO correlation values. FIG. 2 shows a relationship diagram between an ICFO correlation value and an ICFO correlation index. As seen in FIG. 2, the ICFO correlation value appears as an extremely large value (or a maximum value) at a position where the ICFO correlation index is R[0]; the ICFO correlation values corresponding to the neighboring ICFO correlation indices are similar, and are quite different from the ICFO correlation value corresponding to the correlation index R[0] in comparison. The frequency corresponding to the ICFO correlation index R[0] may indicate a frequency offset, e.g., 6000 Hz, and the frequency offset compensation and spectrum reversion circuit 140 in FIG. 1 then compensates the frequency offset of the digital signal according to the indicated frequency offset.
FIG. 3 shows a spectrum diagram of a symbol of a signal based on OFDM modulation. In this example, it is assumed that one symbol includes 8192 subcarriers. In FIG. 3, the upper part shows a common spectrum (or referred to as a forward spectrum), and the lower part shows a reversed spectrum. The reversed spectrum and the forward spectrum are left-right reversed, also known as laterally reversed. That is, a subcarrier originally having a subcarrier number 1 in the forward spectrum appears as a subcarrier (having a subcarrier number 8192) at the end of the reversed spectrum, a subcarrier originally located at the end of the forward spectrum appears at the position of the subcarrier having the subcarrier number 1 in the reversed spectrum, and other subcarriers also appear as laterally reversed regarding the spectrum as the center. The forward spectrum and the reversed spectrum together exhibit a characteristic—for any of the subcarriers, a sum of the subcarrier numbers in the forward spectrum and in the reverse spectrum is a constant value. For example, the circled spectrum components in the forward spectrum and the reversed spectrum have numbers that add up to a sum of 8193.
One reason causing the reversed spectrum is due to different spectrum processing methods at the RF signal transmitter end and at the tuner 110. For example, the transmitter end transmits signals by a reversed spectrum, whereas the tuner 110 processes the RF signal by a processing method for a forward spectrum, hence a reversed spectrum; and vice versa. If the ICFO estimation circuit 170 operates based on an reversed spectrum, a correlation result generated is similar to the relationship diagram in FIG. 2. That is, although a maximum ICFO correlation value is obtained, this maximum ICFO correlation value is still smaller than the maximum ICFO correlation value in the forward spectrum and does not correspond to frequency of a correct carrier frequency offset.
Regarding the correct carrier frequency offset, assume that after another correlation operation performed by the frequency offset compensation and spectrum reversion circuit 140, the ICFO correlation value obtained indicates that the carrier frequency offset is 0. That is, when the ICFO correlation value obtained from again performing the correlation operation indicates that the carrier frequency offset is 0, it means that the OFDM baseband receiver 10 processes the correct spectrum. Conversely, regarding an incorrect carrier frequency offset, assume that after another correlation operation performed by the frequency offset compensation and spectrum reversion circuit 140, the ICFO correlation value obtained indicates that the carrier frequency offset is not 0. That is, when the ICFO correlation value obtained from again performing the correlation operation indicates that the carrier frequency offset is not 0, it means that the OFDM baseband receiver 10 may have processed a reversed spectrum by a processing method for a forward spectrum. When the ICFO estimation circuit 170 discovers that the spectrum is reversed, a control signal is sent to the frequency offset compensation and spectrum reversion circuit 140 to control the frequency offset compensation and spectrum reversion circuit 140 to process the digital signal in the time domain, such that the processed digital signal, after undergoing the FFT circuit 160, displays a forward spectrum in the frequency domain. The processes of the frequency offset compensation and spectrum reversion circuit 140 include swapping the imaginary part and the real part (IQ) swap) or obtaining the complex conjugate of the digital signal. The processed result is further processed by FFT to obtain the reversed spectrum.
When the OFDM baseband receiver 10 processes a forward spectrum, the ICFO estimation circuit 170 at least needs to perform two correlation operations (one for obtaining the carrier frequency offset, and the other for confirming that the compensated carrier frequency offset is 0) to complete the frequency offset compensation. Further, when the OFDM baseband receiver 10 processes a reversed spectrum, the ICFO estimation circuit 170 needs to perform four correlation operations (two for an reversed spectrum and two for a forward spectrum) to complete the frequency offset compensation. As a result, overall efficiency is severely degraded when the OFDM baseband receiver 10 encounters a reversed spectrum.